1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system, such as a copying machine or a laser printer, in which an image is formed by secondarily transferring a toner image to a recording medium from an intermediate transfer body on which the toner image has been transferred, and particularly to a cleaning technique of a transfer part in a secondary transfer portion.
2. Description of the Related Art
Conventionally, as an image forming apparatus using an electrophotographic system, such as the copying machine or laser printer, there is an image forming apparatus constructed such that toner images of yellow, magenta, cyan, black, etc. sequentially formed on a photoreceptor drum are transferred onto an intermediate transfer belt in a state where they overlap with each other, the multiple toner images of the respective colors transferred onto the intermediate transfer belt are transferred to a recording medium at the same time by pressing force and electrostatic attraction force of a backup roll and a secondary transfer roll, and then, the unfixed toner images of the respective colors are fixed onto the recording medium by a fixing device, whereby a color image is formed.
In the image forming apparatus, the secondary transfer roll is brought into press contact with the intermediate transfer belt on which the toner images of the respective colors have been transferred, and the multiple transferred toner images on the intermediate transfer belt are secondarily transferred onto the recording medium, so that the color image is formed. Thus, in the image forming apparatus, when the toner image transferred onto the intermediate transfer belt comes in contact with the secondary transfer roll, the toner is shifted to the secondary transfer roll by the pressing force, and the toner sticking to the secondary transfer roll adheres to the rear surface of the recording medium next conveyed to a secondary transfer position, which becomes a cause of rear surface stain of the recording medium.
For the purpose of preventing such rear surface stain of a recording medium caused by adhesion of toner to a secondary transfer roll, in an image forming apparatus using a transfer roll including a secondary transfer roll, a technique of cleaning the secondary transfer roll or the like has been already proposed in Japanese Patent Unexamined Publication No. Hei. 8-272235, No. Hei. 8-328401, and No. Hei. 9-6146.
An image forming apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 8-272235 includes an image carrier which electrostatically holds a toner image, a transfer member which comes in contact with the surface of the image carrier and applies a transfer bias, and a bias application part which sequentially applies bias currents having different polarities to the transfer member when a transfer material does not exist at a transfer position. In the image forming apparatus, the bias application part applies the same polarity current having the same polarity as the toner constituting the toner image, and then, applies the opposite polarity current having the polarity opposite to the toner and having a current value of an absolute value not lower than an absolute value of the same polarity current.
An image forming apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 8-328401 includes an image carrier which electrostatically holds a toner image, a transfer member which comes in contact with the surface of the image carrier and applies a transfer bias, and a bias application part which applies the transfer bias to the transfer member when a transfer material passes between the image carrier and the transfer member so as to transfer the toner image formed on the image carrier to the transfer material, and applies a current having a polarity opposite to the charged polarity of the toner to the transfer member after applying a current having the same polarity as the charged polarity of the toner at the time of cleaning when the transfer material does not exist at a transfer position. In the image forming apparatus, a bias having the same polarity as the transfer bias is applied to the transfer member at least in the period from the start of first image formation immediately after the cleaning to the application of the transfer bias.
An image forming apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 9-6146 includes a photoreceptor on which a toner image corresponding to an image to be formed is formed, a primary transfer member which transfers the toner image on the photoreceptor onto an intermediate transfer body, a secondary transfer member which is provided to be freely pressed on and separated from the intermediate transfer body and transfers the toner image on the intermediate transfer body onto a recording sheet, and an intermediate transfer body driving part which rotates and drives the intermediate transfer body. The image forming apparatus further includes a cleaner blade which is provided to be freely pressed on and separated from the intermediate transfer body and removes the toner image on the intermediate transfer body, a secondary transfer member holding part which holds the secondary transfer member while selectively changing a press contact state and a separating state to the intermediate transfer body, a cleaner blade holding part which holds the cleaner blade while selectively changing a press contact state and a separating state to the intermediate transfer body, a paper jam detection part which detects a paper jam in a recording sheet conveying passage, a paper jam release detection part which detects that a paper jam state is released, a voltage application part which applies a voltage to the secondary transfer member, and a control part which controls the operation of the respective holding parts, the driving part, and the voltage application part on the basis of detection signals by the respective detection parts. The control part performs such control that after the release of the paper jam state is detected, the cleaner blade is held in the press contact state to the intermediate transfer body, the secondary transfer member is held in the separating state to the intermediate transfer body, cleaning on the intermediate transfer body is performed while the intermediate transfer body is caused to make at least one rotation, the secondary transfer member is held while the press contact state to the intermediate transfer body is changed after the intermediate transfer body is cleaned, a voltage is applied to the secondary transfer member, and cleaning of the secondary transfer member is performed while the secondary transfer member is caused to make at least one rotation.
However, the foregoing prior art has the following problems. That is, in the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-272235 or No. Hei. 8-328401, at the time of cleaning when the recording medium does not exist at the transfer position, the voltage having the same polarity as the charged polarity of the toner is applied to the transfer part, and then, the voltage having the polarity opposite to the charged polarity of the toner is applied, so that the toner sticking to the transfer part is completely reversely transferred onto the image carrier, and the rear stain of the recording medium is prevented. In the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 9-6146, the contact and separation control of the cleaner blade and the secondary transfer member is performed, so that cleaning of the toner remaining on the intermediate transfer body and the secondary transfer member after the release of the paper jam can be effectively performed, and in the image forming operation carried out after the release of the paper jam, the stain such as fogging or stripes due to the remaining toner is not produced, a clear image can be formed, and the image quality of the formed image is improved.
However, there has been a problem that even if a constant cleaning voltage is applied to the transfer part, if consideration is not paid to the relation between the mechanical adhesion force of toner to the surface of the secondary transfer roll or the like as the transfer part and the adhesion force of toner to the surface of the image carrier with which the secondary transfer roll comes in contact, it is impossible to effectively remove the toner adhered to the surface of the secondary transfer roll or the like as the transfer part, and the surface of the secondary transfer roll can not be finely cleaned.
As a result, in the image forming apparatus such as a color copying machine, the cleaning part for applying the cleaning voltage to the secondary roll is made auxiliary, and the blade for cleaning the surface of the secondary transfer roll is used as a main cleaning part. Thus, there has been a problem that the surface of the secondary transfer roll is abraded by the press contact of the blade, and the life becomes short. Especially in the case where a color image is formed, the amount of toner to be cleaned is about 4 times as large as that of a black-and-white image, so that the cleaning part for applying the cleaning voltage to the secondary transfer roll is insufficient, and the load by the blade is large.
The present invention has been made to solve the problems of the prior art and provides an image forming apparatus which can finely clean a transfer roll at all times by regulating the relation between a mechanical adhesion force of toner to the surface of a transfer roll and a mechanical adhesion force of toner to the surface of an image carrier with which the transfer roll comes into press contact.
In order to solve the problems, according to an aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that at the time of cleaning the transfer roll, both an electrostatic adhesion force and a mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier are set larger than an electrostatic adhesion force and a mechanical adhesion force of the charged colorant to the surface of the transfer roll.
Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, a part which sets the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll includes a part which applies a cleaning bias voltage having a polarity opposite to the transfer bias voltage to at least one of the backup roll and the transfer roll.
Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, a part which sets the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll includes a part which sets surface energy of the belt-like image carrier larger than surface energy of the transfer roll.
Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, a contact angle of water on the surface of the belt-like image carrier is set to 70xc2x0 to 80xc2x0, and a contact angle of water on the surface of the transfer roll is set to 85xc2x0 to 100xc2x0.
Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled so that a potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning.
Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled according to a system resistance between the backup roll and the transfer roll.
Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled according to an environmental variation.
Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled according to a use history of the image forming apparatus.
Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and a resistor of a predetermined value corresponding to a system resistance between the backup roll and the transfer roll is provided between a cleaning bias power supply and a transfer portion so that a potential difference between the belt-like image carrier and the transfer roll becomes a value suitable for cleaning.
Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, the resistance value of the resistor is set so that an optimum cleaning bias can always be applied against a change of the system resistance of the transfer portion.
According to the present invention, at the time of cleaning the transfer roll, both the electrostatic adhesion force and the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier are set larger than the electrostatic adhesion force and the mechanical adhesion force of the charged colorant to the surface of the transfer roll. Thus, the charged colorant adhered to the surface of the transfer roll can be certainly shifted from the surface of the transfer roll to the surface of the belt-like image carrier by both the electrostatic adhesion force and the mechanical adhesion force, it becomes possible to finely clean the transfer roll at all times, and an excellent cleaning property of the transfer roll can be assured without using a specific cleaning part.
Besides, according the present invention, in the foregoing image forming apparatus, the part which sets the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll includes the part which applies the cleaning bias voltage having the polarity opposite to the transfer bias voltage to at least one of the backup roll and the transfer roll. Thus, by setting the cleaning bias voltage applied to at least one of the backup roll and the transfer roll, the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier can be easily and certainly set larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll.
Besides, according to the present invention, in the foregoing image forming apparatus, the part which sets the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll includes the part which sets the surface energy of the belt-like image carrier larger than the surface energy of the transfer roll. Thus, by suitably setting the surface energy of the belt-like image carrier and the transfer roll, the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier can be easily and certainly set larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll.
Besides, according to the present invention, the image forming apparatus includes the belt-like image carrier which holds the visible image with the charged colorant and is circularly moved, the transfer roll which is disposed at the front surface of the image carrier to be brought into press contact through the recording medium and collectively transfers the visible image on the image carrier onto the recording medium, the backup roll which is disposed opposite to the transfer roll to be brought into press contact with the rear surface of the belt-like image carrier and forms the predetermined width transfer nip region to the transfer roll, and the transfer bias application part which applies the transfer bias voltage to at least one of the backup roll and the transfer roll, in which the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning. Thus, even in the case where an environmental variation or a change with time occurs, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times.
Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to the system resistance between the backup roll and the transfer roll. Thus, by actually measuring the system resistance between the backup roll and the transfer roll and controlling the output value of the cleaning bias voltage according to the measurement value of the system resistance, even in the case where an environmental variation or a change with time occurs, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times.
Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to the environmental variation. Thus, even in the case where an environment such as temperature or humidity is changed, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times.
Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to the use history of the image forming apparatus. Thus, even in the case where the system resistance of the transfer part is changed with the passage of time, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times.
Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the resistor of the predetermined value corresponding to the system resistance between the backup roll and the transfer roll is provided between the cleaning bias power supply and the transfer portion so that the potential difference between the belt-like image carrier and the transfer roll becomes the value suitable for cleaning. Thus, by the simple structure that the resistor of the predetermined value corresponding to the system resistance between the backup roll and the transfer roll is provided between the cleaning bias power supply and the transfer portion, the potential difference between the belt-like image carrier and the transfer roll can be made the value suitable for cleaning, and even in the case where an environmental variation or a change with time occurs, it becomes possible to finely clean the transfer part at all times.
Besides, according to the present invention, in the foregoing image forming apparatus, the resistance value of the resistor is set so that the optimum cleaning bias can always be applied against the change of the system resistance of the transfer portion. Thus, even in the case where the environmental variation or the change with time occurs, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times.